Our earlier work with chicken and mouse model systems indicate that the activation of the myb oncogene involves specific deletions in the coding regions of the proto-oncogene. Both amino and carboxyl-terminal deletions were observed in both systems. Such deletions lead to the synthesis of truncated myb protine by the activated forms when compared to the normal gene products. It is not clear at this point whether these deletions are essential for the oncogenic activation of the proto-oncogene or if enhanced expression of the normal myb gene alone is adequate for transformation by this gene. We propose to study the mechanism of activation of chicken and mouse c-myb genes by constructing retroviral expression vectors containing the entire c-myb genes or a c-myb gene with specific deletions in its 5' and/or 3' sequences. We also propose to isolate large amounts of the normal and truncated versions of this protein using bacterial and yeast expression vectors. Following the purification of the recombinant myb proteins, we propose to carry out experiments aimed at understanding the biochemical differences between normal and truncated myb proteins.